This study examines the thermodynamic behaviour of coprecipitation reaction for Ni1-x-yCoxMny(OH)2 precursors that is necessary for preparation of cathode materials, LiNi1-x-yCoxMnyO2, of Li-ion batteries under high-ionic-strength process solutions. To capture non-ideal solution behavior while maintaining practical parameter availability, a hybrid activity framework is proposed. Pitzer interactions are applied to dominant background ions including Na+, NH4+, and SO42-, while Davies and extended Debye–Hückel corrections are employed for metal-ammine and metal-hydroxo complexes. Using a consistent activity-based formulation, the total solubilities of Ni2+, Co2+, and Mn2+ are calculated and visualized exclusively within the precipitation-relevant regime. Under the industrially relevant pH range (9 - 12), optimum pH ranges are identified based on inter-metal disparity and precipitation margin as a function of composition of precursors and total ammonia concentration. Compared with concentration-only or Davies-only approaches, the hybrid framework yields a more conservative and practically relevant operating window for pH and ammonia control. Although experimental calibration is not included, the framework provides an activity-consistent solubility map suitable for extension with refined equilibrium data.